In recent years, as the speed of personal computers and workstations increases, the operation speed of computing LSI (Large Scale Integrated circuit) such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor), and a GPU (Graphics Processing Unit) has steadily increased. Such an LSI generates more heat at a higher operation speed, i.e., a higher clock frequency. The heat from the LSI may lead the LSI itself to thermal runway or may have an impact on peripheral circuits. Therefore, it is very important to appropriately cool the LSI.
An exemplary technique for cooling the LSI is an air cooling method using a cooling fan. According to this method, for example, a cooling fan is installed to face the surface of the LSI and flow cold air onto the surface of the LSI.
If a motor for driving the cooling fan locks due to foreign substance caught in the fan, an excess current may flow into the coil or semiconductor element, impairing reliability of the device. To solve this problem, a lock protection circuit is used to stop the supply of power to the motor coil if the motor stops.
According to a technique, if a rotation sensor for detecting the rotation state of the motor detects that the motor has stopped rotating, an automatic resumption signal E is generated until the motor resumes rotating. The automatic resumption signal E is a signal which alternates ON for about 0.5 sec and OFF for about 3 sec. That is, if it is detected that the motor has stopped rotating, the motor is repeatedly driven for about 0.5 sec with an idle period of about 3 sec interposed therebetween.
According to the above technique, the lock protection function operates not only when the motor locks but also when the motor stops according to the instruction of a control input signal. Therefore, if the lock protection function operates and a signal for rotating the motor is input from the outside, the motor does not rotate until the automatic resumption signal E is turned on. That is, after the motor stops according to the instruction of the control input signal, a time lag occurs before the motor restarts rotating upon detection of a temperature rise of the object to be cooled. This time lag may result in difficulty in controlling the temperature.
Another technique restarts the rotation of a motor immediately after the motor stops according to the instruction of a control input signal. According to this technique, when the control input signal for the fan motor instructs the motor to stop for a predetermined period of time, the lock protection circuit is disabled (turned off). Accordingly, it is possible to expedite the time to re-drive the motor after the motor stops according to the instruction of the control input signal.